This invention relates to a method of forming a fluorescent screen for cathode-ray tubes.
In cathode-ray tubes, for instance color picture tubes of a shadow mask type, the fluorescent screen comprises phosphor picture elements of three colors, red, blue and green which are arranged regularly on the inner surface of the panel in a predetermined fashion, for instance, of dots or of stripes. In recent years, there has been a popular trend to provide non-luminuous light absorbing material layer (back substance) between one phosphor picture element and another. There has also been used phosphor with pigments wherein pigment is deposited on phosphor particles.
The method of forming a fluorescent screen will now be described in respect of a color picture tube having a non-luminous light absorbing substance layer, that is, popularly known as a black matrix (BM) type color picture tube. More particularly, on the inner face of the panel is formed a photoresist film which is then dried, exposed through a shadow mask and developed to form a photoresist layer of a matrix pattern. Then, a coating of graphite is applied on the photoresist layer and etched to form a BM pattern film. The inner face of the panel and the BM pattern film are thereafter covered by a thin undercoat layer of water soluble polymeric material.
Next, the portion of the panel is successively coated with respective phosphor slurries of green blue and red and exposed to light via the shadow mask separately for each color. Unexposed portion is dissolved by, for instance, developing with warm water to form a phosphor layer of phosphor picture elements of a desired pattern. The phosphor layer is coated with an organic resin film over which is vapor deposited a thin aluminium film. In order to thermally decompose all the organic substances contained in layers of a fluorescent screen, the panel is baked after formation of the thin aluminium film so as to form the fluorescent screen for the color picture tube.
The phosphor screen coating process comprising the above steps has such a disadvantage that phosphor tends to peel off and tear in the developing process using warm water and the like if the adhesive force between the inner face of the panel and the fluorescent film is insufficient.
On the other hand, when the adhesion between the inner face of the panel and the fluorescent film is excessively strong, phosphor will remain in the unexposed portion, causing the color purity to become degraded. The usual method to effectively deposit a phosphor layer on the inner face of the panel is to form a thin undercoat of water soluble polymeric substance on the clean glass panel inner face as described above. Typically, the method comprises washing the inner face of the panel with an aqueous solution of hydrofluoric acid, rinsing with deionized water, coating with diluted polyvinyl alcohol solution at a concentration of about 0.2 to 0.5% by weight and drying it. As a result, an extremely thin undercoat layer of polyvinyl alcohol, almost of the thickness of monomolecular film, is formed upon the inner face of the panel glass.
It is known that this layer improves the adhesive property of a phosphor coating which comprises phosphor particles and polyvinyl alcohol or it is imparted with photosensitivity by dichromic acid which is to be applied in the ensuing step. To further improve the adhesive property, there are available such a method as to improve the coating on the phosphor particle surface, either alone or in combination with the undercoat, a method wherein a sensitizer or silica, silane coupling agent or emulsion which turns into a film by heating, is added to the phosphor slurry or a method wherein ultraviolet rays are irradiated on the outside surface of the panel for a predetermined time at or after the exposure via the shadow mask.
When trying to improve the adhesive property of the phosphor, one finds that phosphor remains on the unexposed area of the panel glass during the development step after exposure, thereby causing cross-contamination and degradation in color purity of the fluorescent screen. Further, phosphor remains on such regions other than the effective area in the unexposed portion, for instance, inside face of the panel glass skirt. And when a color picture tube with such a fluorescent screen is operated, the phosphor adhered to the region outside of the effective area luminesces when the electron beams from the electron gun are overscanned or scattered, impairing the quality of the picture tube.
This presents various structural problems in the picture tube of the type wherein a side portion of the panel is exposed in use from the television set, namely of push-through type.
For eliminating the phosphor remaining on the unnecessary area there have been proposed such methods as a method to adjust conditions for development after exposure such as spray pressure, time or temperature of the warm water, a method to add a certain additive to a slurry containing phosphor and photosensitive binding agent, so-called phosphor slurry, and a method to reduce the concentration and polymerization degree of the binding agent such as polyvinyl alcohol. However, it has not been possible to remove phosphor completely without causing peeling-off and tear of the phosphor layer. Moreover, the inner face of a panel has to be wiped manually or mechanically by a device in order to remove the remaining phosphor outside the effective area of the panel. This has caused various difficulties, resulting in inferior finishing on the panel side or reduced efficiency of operation. Another prior art fluorescent screen forming method has been proposed in which an undercoat layer containing water insoluble organic polymer particles is formed with a film weight of 0.08 to 0.8 mg/cm.sup.2. This method can greatly improve the adhesive property of the phosphor film of the exposed area. However, it has been proven by experiments conducted by inventors of this invention that this method has such fatal drawbacks in that the thick undercoat applied causes random reflexion inside and outside of the panel glass at the time of exposure, giving rise to a phenomenon similar to the so-called dark reaction wherein an area outside the predetermined area is exposed to light and thus leaving phosphor on the unexposed region and generating, after the metal back process, that is, during the baking process after the vapor deposited aluminium film formation, blisters on the aluminium film.
The method utilizing the prior-art undercoat layer has critical drawbacks in that the dark reaction occurs depending on the temperature of the panel glass during or after drying following phosphor slurry coating, and atmospheric conditions at the time of exposure such as the temperature and the humidity.